Lecture 20/gas exchange I

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b. major problems
i. slow rate of diffusion of O₂in water
ii. lower O₂solubility in water
iii. more work required to move water
iv. High water temperature

2. Adaptive features
a. Structure
Fish gills consist of hundreds of leaf-shaped gill filamentsfolded into rows oflamellea. The surface of the lamellae made up of flattened epithelial cells. Water passes between these flat plates.

b. Ventilation42.19
To deal with the lower O₂content, fish take water in through their mouths and pump it out over the gills. This increases the supply of fresh water, bringing in O₂and removing CO₂. However, water is much harder to move than air, so this requires an output of energy.

c. Countercurrent exchange - blood flow is opposite to water flow

42.20

The flow of water across the gills is unidirectional. The flow of blood through the gills is also unidirectional and in the opposite direction. This is very important. If the blood flow was in the same directionas the water, O₂would be exchanged until the concentrations were the same. After that diffusion won't result in a net transfer, so the maximum that could be exchanged is 50% of the O₂. By moving the blood in the opposite direction, the blood always has a lower concentration of O₂than the water and greater than 80% of the O₂can be transferred.

Countercurrent flow is very powerful strategy for maximizing exchange; look for it later in the semester.

Q. Which variables in Fick's law do these features affect?
- gill structure (flat, folded lamellae) increase surface areaand decrease diffusion path length
- ventilation and countercurrent exchange increase concentration difference


	b. major problems i. slow rate of diffusion of O₂in water ii. lower O₂solubility in water iii. more work required to move water iv. High water temperature
2. Adaptive features a. Structure Fish gills consist of hundreds of leaf-shaped gill filamentsfolded into rows oflamellea. The surface of the lamellae made up of flattened epithelial cells. Water passes between these flat plates. b. Ventilation42.19 To deal with the lower O₂content, fish take water in through their mouths and pump it out over the gills. This increases the supply of fresh water, bringing in O₂and removing CO₂. However, water is much harder to move than air, so this requires an output of energy.
	c. Countercurrent exchange - blood flow is opposite to water flow	42.20
The flow of water across the gills is unidirectional. The flow of blood through the gills is also unidirectional and in the opposite direction. This is very important. If the blood flow was in the same directionas the water, O₂would be exchanged until the concentrations were the same. After that diffusion won't result in a net transfer, so the maximum that could be exchanged is 50% of the O₂. By moving the blood in the opposite direction, the blood always has a lower concentration of O₂than the water and greater than 80% of the O₂can be transferred. Countercurrent flow is very powerful strategy for maximizing exchange; look for it later in the semester. Q. Which variables in Fick's law do these features affect? - gill structure (flat, folded lamellae) increase surface areaand decrease diffusion path length - ventilation and countercurrent exchange increase concentration difference